This invention generally concerns a power generation process using a gas turbine for conversion of the energy of gas, and power plant for carrying out the process. More particularly the invention is concerned with such a process and apparatus which employ gas produced by the gasification of coal in a gasifier.
In coal gasification installations, a low- to medium-calory fuel gas is produced from coal under elevated pressure and at elevated temperature, using an oxygen-bearing medium such as air or steam and oxygen as a gasification agent. The fuel gas produced in that way is particularly suitable for use in combined gas/steam turbine power generating plants. By virtue of the combination of gas and steam turbines, such power generating plants generally have a level of efficiency which is substantially higher than that of conventional power plants with flue gas desulfurisation and denitrogenisation.
German laid-open application (DE-OS) No. 35 01 456 discloses a method of reducing the SO.sub.2 and NO.sub.x content of gases which are produced in the gasification of coal and which undergo combustion after desulfurisation in a turbine. In order to achieve a reduction in the amount of NO.sub.x discharged with the exhaust gas and to achieve an increase in the output and performance of the power generating installation by virtue of a lower level of compression energy for the combustion air, it is proposed that combustion of the desulfurised gas take place in the presence of a CO.sub.2 /O.sub.2 mixture. The use of CO.sub.2 on the one hand lowers the combustion temperature and on the other hand reduces the NO.sub.2 problem as a practically No.sub.2 -free exhaust gas is produced. In comparison with air, the amount of CO.sub.2 required is lower so that the energy requirement for the CO.sub.2 -compression step can be reduced.
The known method suffers from a series of disadvantages. Firstly, it is highly uneconomical to operate a combustion process with pure oxygen. Furthermore, recycling crude CO.sub.2 by way of the compressor results in a loss in efficiency of the combination plant. Although the recycling of CO.sub.2 to the combustion process means that thermal NO.sub.x is not produced, or is produced only in small amounts, on the other hand, due to reaction-kinetic effects, the nitrogen in the fuel is reacted, even at the low temperatures involved, to produce nitrogen oxides which rapidly leave the system so that it becomes impossible to arrive at an equilibrium condition. Reaction-kinetic effects of that kind are predetermined for example by the residence time and the reaction mechanism. The reaction of NH.sub.3 or HCN with O.sub.2 does not stop at the level of N.sub.2 but goes on in the direction of NO and NO.sub.2, which is what is precisely to be avoided.
Finally, the known method makes no provision whatsoever for the removal of dust and there are therefore doubts about the technical viability of that method because a dust-laden gas cannot be used and allowed to expand in a gas turbine.
At the same time however, the cleaning of hot gas is known. In such a procedure, the gas is cleaned of dust by means of temperature-resistant and pressure-resistant ceramic filter cartridges and is desulfurised in fixed bed reactors or fluidised bed reactors by reaction with metal oxides. The resulting metal sulfides are then regenerated again. Such a hot gas cleaning process has advantages over gas cleaning processes using low temperatures, because in that way the overall degree of efficiency of power generating processes with a gas turbine can be increased. However, hitherto the known methods of cleaning the hot gas which is in a pressurised condition did not afford any way of adequately removing the nitrogen compounds such as ammonia and hydrogen cyanides which are contained therein. Such compounds must be removed from the exhaust gases after expansion of the gas which has undergone combustion, in the gas turbine, in order to prevent environmental pollution, and that is an expensive operation. A de-NO.sub.x installation which is disposed downstream of the gas turbine requires a relatively high pressure drop because the effective gas volume to be cleaned is about 150 to 200 times greater in comparison with the gas before it has undergone expansion and combustion. In addition, an expensive temperature control arrangement is required in the waste heat and steam generating system. Therefore such a procedure runs counter to the advantages in regard to increasing the level of efficiency, which are achieved with hot gas dust removal and desulfurisation.